Indirectly heated cathode (IHC) ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the ion source chamber. The cathode is disposed at one end of the ion source chamber. A repeller is typically disposed on the end of the ion source chamber opposite the cathode. The repeller may be biased so as to repel the electrons, directing them back toward the center of the ion source chamber. In some embodiments, a magnetic field is used to further confine the electrons within the ion source chamber. The electrons cause a plasma to be created. Ions are then extracted from the ion source chamber through an extraction aperture.
The ion source chamber is typically made of an electrically conductive material, which has good electrical conductivity and a high melting point. The ion source chamber may be maintained at a certain electrical potential. Additionally, the cathode and the repeller are disposed within the ion source chamber, and are typically maintained at electrical potentials that are different from the ion source chamber.
Ion source chambers are often lined to extend the life of the ion source. For example, tungsten liners are often installed in ion source chambers. These tungsten liners are subjected to ion bombardment and extreme temperatures. As these tungsten liners decay, they may be replaced without having to replace the entirety of the ion source chamber. Tungsten or other high temperature refractory metals are often selected due to its electrical conductance.
Tungsten liners are also thermally conductive, so the heat of the plasma is typically transferred to the ion source chamber. However, in certain embodiments, it may be advantageous to maintain a temperature difference between the plasma and the ion source chamber. Specifically, ion source chamber temperature is a critical parameter particularly for species such as carbon and boron trifluoride, where deposition of unwanted materials builds up on surfaces affecting uniformity, lifetime and cost.
Therefore, it would be beneficial if there were a liner which allows a temperature gradient to be introduced between the plasma and the ion source chamber. Further, it would be advantageous if these liners were low cost and easily replaced.